Why bother supporting old mysql extension: ADOdb still supports it and ADOdb and Flyspray support also older PHP versions. So no urgent reason yet to drop and other stuff has higher priority.

Adding the pdo_mysql driver code part to includes/class.database.inc.php is just experimental and added by me as an example. The problem is, that the 'drivers' of ADOdb are not equal in features and quality (as they should) and the xmlschema03 install and upgrades must result consistently in same quality results for each db connection type it official supports. Even for postgresql comes with our own setup/upgrade/1.0/datadict-postgres.inc.php file that patches ADOdb. Thats why I'm currently focusing on pushing/improving ADOdb project https://github.com/ADOdb/ADOdb for cross db tests.

That is also the reason I do 1.0-rcX releases instead 1.0.1, 1.0.2, .. releases. Just fear to break with the, must get confidence with tests before doing setup/upgrade/1.0.1 directory.